In a 3D display system, a display is used to alternately show a right image and a left image and a viewing apparatus having a right shutter and a left shutter alternately opened to allow a viewer to see the right image with the right eye and the left image with the left eye through the opened shutter. The viewing apparatus is usually shaped like a pair of glasses to allow the viewer to wear it while viewing the display.
It is essential that the shutters on the viewing apparatus open and close at the right times, in steps of the showing on the display.